Mitt Romney said Wednesday that a mandate in President Obama’s signature health-care law is “a tax,” contradicting a position his campaign staked out this week and belatedly getting in line with many other Republican leaders.

The presidential candidate said in an interview with CBS News that he accepts last week’s Supreme Court decision, which upheld the legislation. The law’s individual mandate, which imposes a fine on those who do not obtain health insurance, is a tax, the court ruled, and is therefore constitutional under Congress’s power to assess taxes.

On Monday, senior Romney adviser Eric Fehrnstrom said the former Massachusetts governor rejected the court’s characterization and believed that the individual mandate was a penalty, not a tax.

Although Romney’s comments on Wednesday opened him up to renewed criticism from Democrats that he had changed positions for political expediency, campaign spokeswoman Amanda Henneberg maintained that his remarks “were in line” with what Fehrnstrom said Monday.

Romney said he disagrees with the court’s ruling, and aides said he still thinks the individual mandate is an “unconstitutional penalty,” but he said he accepts that the court’s majority opinion is now the law of the land.

“The Supreme Court has the final word, and their final word is that Obamacare is a tax,” Romney told CBS. “So it’s a tax. They decided it was constitutional. So it is a tax, and it’s constitutional.”

Later, Romney said: “While I agreed with the dissent, that’s taken over by the fact that the majority of the court said it’s a tax, and therefore it is a tax. They have spoken. There is no way around that.”

Health-care reform has been a difficult issue for Romney, given the many similarities between the Massachusetts legislation he championed and signed as governor and the federal law that has become one of the most inflammatory issues for conservative voters. The Supreme Court’s ruling made the matter even trickier for Romney.

By saying the mandate is a tax, Romney seemed to acknowledge that the provision in the Massachusetts law that, like Obama’s federal law, fines people who don’t buy health coverage also is a tax. But he argued that is not the case.

Had Romney not called the federal mandate a tax, he would have been at odds with most other Republican leaders in the country, who have seized on last week’s ruling to open a fresh line of attack against Obama and the legislation they call “Obamacare.”

In the CBS interview, Romney signaled that he would campaign on the Supreme Court ruling and he began to brand Obama as imposing new taxes on the middle class.

“They concluded it was a tax, that’s what it is, and the American people know that President Obama has broken the pledge he made. He said he wouldn’t raise taxes on middle-income Americans,” the Republican said.

Romney’s comments Wednesday interrupted what seemed like a perfectly scripted Fourth of July for the candidate, who emerged for the first time from his family vacation at his lakefront compound in Wolfeboro to march in the town’s annual parade.

Mitt and Ann Romney, their five sons and many of their 18 grandchildren walked along Main Street, shaking hands, waving American flags and gulping lemonade as a camera crew shot footage for commercials.

All year on the campaign trail, Romney has vowed to lead the effort to repeal Obama’s health-care law, and he argued after the court ruling that the only way to do so is to elect a new president.

In his CBS interview, Romney offered reasoning that may prove difficult for many voters to follow.

When correspondent Jan Craw­ford asked whether his Massachusetts law also is a tax, he said: “The chief justice in his opinion made it very clear that at the state level, states have the power to put in place mandates. They don’t need to require them to be called ‘taxes’ in order for them to be constitutional. And as a result, Massachusetts’ mandate was a mandate, was a penalty, was described that way by the legislature and by me, and so it stays as it was.”

Since his opponents in the 2008 GOP presidential primaries branded him a flip-flopper, Romney has gone to great lengths to try to avoid that label in this campaign. But Obama’s reelection campaign officials saw in Romney’s comments Wednesday an opening, and they rushed out a statement saying that the Republican had “contradicted his own campaign, and himself.”

“First, he threw his top aide Eric Fehrnstrom under the bus by changing his campaign’s position and calling the free rider penalty in the President’s health care law — which requires those who can afford it to buy insurance — a tax,” Obama campaign spokesman Danny Kanner said. “Second, he contradicted himself by saying his own Massachusetts mandate wasn’t a tax — but, Romney has called the individual mandate he implemented in Massachusetts a tax many times before. Glad we cleared all that up.”

Republican strategists said the public debate over whether the mandate is a penalty or a tax will mean little to voters in November.

“I think it means absolutely nothing or less than nothing,” said Alex Castellanos, a top adviser to Romney in 2008 who is no longer affiliated with him. “Whether it’s a tax or a mandate, that’s not what people are going to look at to try and find a difference between these two candidates.”

Greg Mueller, an adviser to conservative groups, said, “What­ever he wants to call it is one thing,” adding: “At the end of the day, Romney’s going to be the candidate ready to repeal — branch and root — Obamacare and stop this massive tax increase, and Obama’s going to be the guy defending it.”